The present invention relates to a method of measuring the sensitization of a structural material of a plant and a system for the same, and, in particular to a method of plant operation management based upon a sensitization measuring method concerned with the intergranular stress corrosion cracking susceptibility of a plant structural member made of, for example, a stainless steel. The present invention can be applied in any field where monitoring of the intergranular stress corrosion cracking of a structural member due to sensitization is possible, e.g. a thermal power generating plant and a chemical plant.
Methods of measuring the sensitization have recently been compiled by a subcommittee 63-2 of Boshoku Kyokai in a report published in "Boshoku Gijyutsu (Anti-corrosion technology), Vol. 39 N. 11, 1990). Among these prior art methods, a non-destructive sensitization measuring method of the type into which the method of the present invention is classified is described in "A method of measuring the electrochemical reactivation rate of stainless steel", Japanese Industrial Standard JIS GO 580 (1986). This technique is generally referred to as EPR 25 (Electrochemical Potentio-kinetic Reactivation) method. The nondestructive electrochemical sensitization measuring method is classified into this EPR method. In this method anode polarization is performed measuring method is classified into this EPR method. In this method, anode polarization is performed starting from the natural electrode potential. Immediately after reaching +0.3V using a saturated calomel reference electrode, the potential is swept in an opposite direction. After reactivation, the sweeping is terminated at a potential at which the anode current again becomes zero.
The result is determined by the following formula. The value is rounded to the first decimal place according to JIS Z 8401.
Reactivation rate (%)=(maximum anode current density in active mode in the forward path) / (maximum anode current density in active mode in the return path).times.100.
In the EPR method, the amount of sample liquid and the potential sweeping speed are prescribed as not less than 200 ml and 100.+-.5 mV/min, respectively. Accordingly, reduction in size of the electrochemical cell is limited to 200 ml in these conditions.
In the 33rd discussion on anticorrosion held in Nagano, Japan in 1990 and sponsored by Fushoku Boshoku Kyokai, lecture No. C-201 reports that an increase in anticorrosion current from the grain boundary apparently becomes higher than that in grains by irradiation with a laser beam. However, in this report, the relation between the irradiation with a laser beam and the sensitization of a member has not been studied.
There are a number of scientific literature publications on electrochemical pulse instrumentation. "Electrochemical Methods" published by John Wiley & Sons, Inc. (1980), pp 176 to 206 mainly describe principles of reverse pulse, normal pulse, and differential pulse voltammetry in detail. Concerning the square wave voltammetry, quantitative analysis techniques using peak area are reported in Analytical Chemistry, Vol. 36, pp 420 to 424 (1987). However, pulse voltammetry has heretofore been studied from the aspect of an electrical analysis technique of a very small amount of metal ions and as an analysis technique of electrochemical reaction mechanisms. There has been no study to detect the sensitization of structural elements using these pulse voltammetric instrumentation techniques for the management of plant operation. There has been no report that the sensitization of structural members can be detected by using the pulse voltammetric measuring technique.
It is preferable to execute the pulse voltammetry in a pulse mode. In the examples of the present invention which will be described hereafter, normal pulse voltammetry and differential pulse voltammetry give excellent results.
The pulse voltammetry which can be performed in accordance with the present invention includes normal pulse voltammetry, reverse pulse voltammetry, differential pulse voltammetry, differential normal pulse voltammetry, square wave pulse voltammetry, etc.